Electrical power converters are devices for processing electrical power from one form, such as an AC or unregulated DC voltage, into another form, such as one or more regulated DC output voltages. One conventional type of electrical power converter that produces a regulated output voltage is a switching power supply, also commonly referred to as a switch mode power supply or a switched power supply.
Conventional switching power supplies generally include a power switch, a power transformer, and a secondary filter circuit for each output of the converter. The power switch is used to alternately couple a DC or rectified AC voltage across a primary winding of the power transformer, thereby applying a series of square wave voltage pulses to the primary winding. The voltage pulses applied to the primary winding of the power transformer are transformed into a series of voltage pulses across one or more secondary windings. An output circuit for each converter output rectifies these voltage pulses, transferring their electrical energy to an output capacitor coupled to a converter output, which is typically coupled to a load. The power supply generally transfers sufficient energy to such output capacitor to maintain an appropriate output voltage level and supply necessary power to the loads associated therewith.
Regulation of the output voltages of a power converter generally is maintained by a pulse width modulator. Conventional pulse width modulators control the duration of the voltage pulses coupled to the primary winding of the power transformer, thereby controlling the amount of electrical energy transferred to the secondary side of the power transformer. To maintain a selected output voltage with light loads (i.e., loads drawing a relatively small amount of power), the power converter operates at a low duty cycle (i.e. short duration voltage pulses coupled to the power transformer) so that a relatively small amount of electrical energy is transferred to the output capacitors in a given time period. Conversely, for heavy loads (i.e., loads drawing a relatively large amount of power), the power converter operates at a high duty cycle (i.e., relatively long duration voltage pulses coupled to the power transformer) so that a relatively large amount of electrical energy is transferred to the output capacitors in a given time period.
One common type of switching power supply is referred to as a flyback power converter. In a conventional flyback power converter, rectifiers coupled to the secondary windings of the power transformer prevent current from flowing through the secondary windings when voltage is coupled across the primary winding. At the beginning of each switching cycle of a flyback power converter, the power switch turns on and couples a voltage across the primary winding. Consequently, current in the primary winding ramps up, thereby storing energy in the form of a magnetic field or flux in the power transformer. The period of time during which the power switch is on is referred to as the drive cycle or drive period. After the switch is turned off, the voltages across the secondary windings reverse, and the energy stored in the power transformer is released through the secondary windings and filtered to produce the desired output voltages. The period of time during which energy is released from the secondary windings is referred to as the flyback cycle or flyback period. After essentially all stored energy is released, the power switch is again turned on and the switching cycle repeats.
The pulse width modulator of a flyback power converter generally is responsive to a feedback voltage that is a function of the level of an output voltage. To generate an appropriate feedback voltage, the output voltage may be measured directly, provided isolation is maintained between the primary and secondary circuits of the converter. Alternatively, the output voltage may be measured by detecting the voltage across a tertiary winding of the power transformer. It is well known that a tertiary winding of a flyback power converter will have a voltage that is substantially proportional to the secondary winding voltages. The feedback voltage is typically rectified and filtered to produce a regulating voltage that is then coupled to the pulse width modulator. As described below, this feedback voltage is substantially proportional to the output voltages under normal or light load conditions.
It is generally preferable to obtain such a feedback voltage from a tertiary winding of the power transformer rather than by directly measuring an output voltage of a secondary winding. Using a tertiary winding eliminates the need for a separate isolation device (e.g. an opto-coupler or transformer), which would be necessary in the case of direct feedback to maintain isolation between the primary and secondary circuits of the converter. In addition to increasing component count, some isolation devices such as opto-couplers are undesirable in that they commonly cause reliability problems.
A key problem with obtaining the feedback voltage from a tertiary winding, however, is that output voltage regulation is degraded because power losses between the windings of the power transformer and the output terminals of the flyback converter are, in effect, transparent to the feedback mechanism. Under heavy load conditions, the forward voltage drop and recovery loss of the output rectifiers, the "copper losses" of the transformer and the printed circuit board conductors existing between the secondary winding and the output terminal, as well as other well known parasitic losses, are higher than at light loads. These losses result in output voltages that are lower under heavy load conditions.
Another problem that occurs when the feedback voltage is obtained from a tertiary winding is that transformer leakage inductance generally degrades the regulation of the output voltages by introducing a leading edge spike (or spikes) in each cycle of the feedback voltage across the tertiary winding. These spikes are followed by a voltage "plateau" that better represents the voltage across the secondary windings. As is well known, many common transformers used with flyback power supplies have relatively high leakage inductances due to the relatively poor magnetic coupling among the transformer's windings, and therefore such transformers generate high leading edge spikes in the voltage across their tertiary winding. Due to the spikes in the feedback voltage, the regulating voltage rises to a level that is higher than it should be for the actual output voltage level, providing inaccurate feedback to the pulse width modulator and resulting in poor regulation. Adequate regulation of the output voltage is even more difficult to achieve because the magnitudes of the leakage inductance spikes are proportional to the load current drawn from the power converter's output. Thus, the effects of the spikes on regulation of the power converter's output voltages varies for heavy and light loads.
A variety of methods have been developed for reducing the effects of leakage inductance spikes on output voltage regulation where a tertiary winding is used to provide feedback. One way to reduce the effect of leakage inductance spikes on output voltage regulation is to filter the spike out of the tertiary voltage waveform. Filters capable of accommodating the necessary switching frequency, however, are generally complex and expensive.
Another way to reduce the effects of the tertiary voltage spikes is to place spike suppressing coupled inductors in the secondary and the tertiary circuits, as discussed in Leman, U.S. Pat. No. 5,008,794. However, the isolated coupled inductors required in Leman are also relatively expensive components and may be very complex in their construction. Leman also discusses a number of other ineffective means of dealing with leakage inductance spikes.
Finally, a spike "blanking" circuit may be used to eliminate the leakage inductance spikes. In such spike blanking circuits, a transistor switch between the tertiary winding and the pulse width modulator is open during the spike and closed during the "plateau" portion of the feedback voltage. Thus, the regulating voltage actually sensed by the pulse width modulator is not affected by the voltage spikes. As explained in Leman, such circuits have generally been considered undesirable because either a complex algorithm for controlling the opening and closing points of the switch is needed, or the power converter's load range must be restricted in order to provide an accurate regulating voltage to the pulse width modulator. Leman also notes that the analog switch action itself can introduce voltage spikes that affect the regulating voltage sensed by the pulse width modulator.
The above-described methods for dealing with leakage induction spikes have the further disadvantage that they do not provide means for dealing with the abovedescribed degradation of output voltage regulation that occurs under heavy load conditions. As noted above, output voltage regulation is degraded under heavy load conditions because the tertiary winding does not account for relatively large power losses in the secondary circuit.
Accordingly, there is a need for a flyback power converter that provides acceptable output voltage regulation under heavy and light load conditions and compensates for leakage inductance of the power transformer.